1. Field of the Invention
The present invention relates to a method for activating a catalyst for producing propylene copolymers, in advance of copolymerization by which said propylene copolymers are to be prepared, and also to a method for producing the propylene copolymers using a catalyst thus activated.
More particularly, the present invention relates to a method for producing crystalline copolymers consisting of three or more monomer components including propylene as a main component, and also to a method for activating a polymerization catalyst to be employed for the production of the copolymers, in advance of the copolymerization by which the copolymers are prepared. The copolymers obtained by said production method are suitable for producing, by processing, cold-resistant films such as packaging films for use in winter season or freezed products, films which are sealable at low temperatures, heat-shrinkable films, highly transparent films, sheets of multiple layers for blow molding, etc.
2. Description of the Prior Art
The art for shaping polypropylene into films, sheets, etc. is well known. In such shaping, however, there has been a demand for producing polypropylene which is superior in the practical physical properties necessary for the above-mentioned shaped products such as low temperature impact-resistance, mutual heat-sealability at comparatively low temperatures (which will be hereinafter referred to as low temperature heat-sealability), heat-shrinkability at the time of processing into stretched films (which will be hereinafter referred to as heat-shrinkability), etc. For this purpose, such a method is effective and has been practically employed that a small amount of a copolymerizable component other than propylene (which will be hereinafter often referred to as comonomer) is, in advance, mixed with a large amount of propylene at an appropriate time and in an appropriate amount, followed by copolymerization to give the propylene copolymers, which have the above-mentioned practical physical properties improved over homopolypropylene.
However, such improvement in the processing physical properties of polypropylene according to the above-mentioned copolymerization method has still a problem. For example, in case of an ethylene-propylene (two monomer component system) random copolymer obtained according to a method wherein a small amount of ethylene as a comonomer is, in advance, mixed with propylene and polymerization is carried out, with the increase of the content of ethylene in the resulting copolymer, the transparency and rigidity of the shaped product prepared from the copolymer are notably reduced; the end-opening property of a bag form product prepared from the above-mentioned shaped film (which will be hereinafter merely referred to as end-opening property) becomes inferior; and the amount of a soluble (atactic) polymer (having no utility as plastics) formed at the time of preparation of said random polymer notably increases.
Accordingly, the attempt to improve the above-mentioned various processing physical properties according to said random copolymerization of two monomer component system, has restrictions in respect of the physical properties of the resulting polymer as well as the production yield of the polymer.
Further, a method for producing a copolymer of three monomer component system consisting of ethylene, propylene and an .alpha.-olefin is disclosed in Japanese patent application laid-open No. 35487/1974. In case of this method, the resulting polymer is superior in the reduction of the soluble polymer relative to the polymer yield, and the transparency which is one of the processing physical properties of the polymer, etc. However, the amount of the soluble polymer by-produced is still yet large, perhaps due to the fact that the catalyst employed is a usual Ziegler-Natta system catalyst. Further, as for the polymer of three monomer component system obtained according to the above-mentioned production method, the low temperature impact-resistance and the end-opening property which are practical physical properties other than the above-mentioned transparency are insufficient, and further, usual physical properties such as rigidity, Young's modulus, etc. are also insufficient.
The solution of the above-mentioned problem is particularly necessary because of the recent trend of consumer's demand directed gradually to higher class shaped products of polypropylene (particularly the above-mentioned films or sheets) having superior performances and physical properties, and accordingly the standard of evaluation of such shaped products set by the processing makers is now very severe. More concretely, if one of the physical properties of a polypropylene product is superior, but another physical property is inferior to others on an average level, the total evaluation of the product is made based upon said inferior physical property. Accordingly, even though improvement in one or two or more practical physical properties is aimed, other practical or usual physical properties cannot be notably reduced, and on the other hand, in order to carry out the polymerization of propylene smoothly and economically, reduction of the amount of the soluble polymer by-produced, is necessary more than those in the prior art.
In order to solve the above-mentioned technical problem, the present inventors have previously proposed a method for producing a copolymer of three monomer component system consisting of ethylene, propylene and another .alpha.-olefin having 4 or more carbon atoms (Japanese patent application No. 3985/1975) (which method will be hereinafter referred to as prior application method).
The present invention is directed to an improved method over the prior art, that is, a method for producing copolymers of three-monomer component or more-monomer component system (referred to hereinafter as multiple component system), employing a catalyst activated in advance of copolymerization.
After strenuous studies made for much more improving the prior application method, the present inventors have found that a copolymer of multiple component system having superior physical properties to those of the prior application method can be obtained by specializing the catalyst for copolymerization, activating the catalyst in advance of copolymerization, specifying the feeding manner of ethylene at the time of copolymerization, and specifying the proportion of the constituents of the resulting copolymer of multiple component system, and at the same time, the proportion of the soluble polymer by-produced can be further reduced, and thus have attained the present invention.